A polymer electrolyte fuel cell or a hydroelectrolytic cell can be listed as an exemplary electrochemical apparatus employing a solid polyelectrolyte as an ion conductor in place of a liquid electrolyte. The polymer electrolyte fuel cell includes a fuel cell fueled by hydrogen gas and a fuel cell fueled by a mixed solution of a hydrocarbon-based fuel represented by methanol and water. Structurally, an electrode-electrolyte membrane assembly prepared by holding an electrolyte membrane (also referred to as a polymer electrolyte membrane, an ion-exchange membrane, a proton-exchange membrane or a proton-conducting polymer film) having proton conductivity with a pair of electrodes causes oxidative reaction in the first electrode and reductive reaction in the second electrode, to operate as a cell or a hydroelectrolytic cell.
The polymer electrolyte membrane employed therefor must be chemically, thermally, electrochemically and mechanically sufficiently stable along with proton conductivity as a cation-exchange membrane. Therefore, a perfluorocarbon sulfonic acid membrane (fluoric proton-conducting polymer) represented by “Nafion (registered trademark)” by Du Pont, U.S.A. has been mainly used as that usable over a long period. If a membrane of Nafion (registered trademark) is driven under a temperature condition exceeding 100° C., however, the water content in the membrane abruptly lowers and the membrane is remarkably softened. Therefore, the working temperature is disadvantageously limited. When a fluoric proton-conducting polymer membrane is used for a fuel cell fueled by a hydrocarbon-based liquid fuel such as methanol, methanol permeates the membrane to remarkably reduce the performance, leading to a significant problem. Further, it is pointed out that the film is so high-priced as to hinder practicalization.
In order to overcome such problems, various studies are conducted on a polymer electrolyte membrane composed of a non-fluoric proton-conducting polymer prepared by introducing a proton-conducting functional group such as a sulfonic acid group or a phosphonic acid group into an aromatic hydrocarbon-based polymer as a substitute for the fluoric proton-conducting polymer. As to the polymer skeleton, an aromatic compound such as aromatic polyarylene, aromatic polyarylene ether ketone or aromatic polyarylene ether sulfone can be captured as a prospective structure in consideration of heat resistance and chemical stability, and a structure obtained by sulfonating polyaryl ether sulfone (refer to Journal of Membrane Science) (Netherlands), 1993, Vol. 83, pp. 211-220 (Non-Patent Document 1) and Specification of U.S. Publication No. 2002/0091225 (Patent Document 2), for example) and a structure obtained by sulfonating polyether ether ketone (refer to Japanese Patent Laying-Open No. 6-93114 (Patent Document 2), for example) are reported.
A polymer electrolyte membrane composed of the aforementioned non-fluoric proton-conducting polymer is regarded as prospective due to such advantages that the film is less deformed under a high temperature dissimilarly to a fluoric proton-conducting polymer membrane and less permeated by methanol upon application to a fuel cell fueled by a liquid fuel such as methanol and a cost expectedly lower than that of the fluoric proton-conducting polymer. For the future, development making the best use of the characteristics of each polymer is expected.
The electrodes used in the aforementioned electrode-electrolyte membrane assembly prepared by stacking the electrodes on the electrolyte membrane are generally prepared by applying a catalyst ink obtained by mixing a composition prepared by dissolving or dispersing a fluoric proton-conducting polymer in solvent or the like and a catalyst suitable for fuel cell reaction with each other onto gas diffusion layers or films and removing the solvent. Thereafter the electrodes are transferred to the electrolyte membrane, thereby forming the electrode-electrolyte membrane assembly (refer to Japanese Patent No. 3523853 (Patent Document 3), for example). A method directly applying a catalyst ink onto an electrolyte membrane or indirectly applying the former to the latter with a spray or the like is also studied.
Various methods are studied for preparing an electrode-electrolyte membrane assembly with an electrolyte membrane composed of a non-fluoric proton-conducting polymer. The non-fluoric proton-conducting polymer has properties different from those of the fluoric proton-conducting polymer, and hence it is necessary to improve bondability between electrodes and the electrolyte membrane. For example, each of Japanese Patent Laying-Open No. 2003-317749 (Patent Document 4), Japanese Patent Laying-Open No. 2003-317750 (Patent Document 5), Japanese Patent Laying-Open No. 2004-55522 (Patent Document 6) and Japanese Patent Laying-Open No. 2003-249244 (Patent Document 7) shows a method of forming an electrode-electrolyte membrane assembly by applying and drying a non-fluoric proton-conducting polymer solution to and on electrodes (including catalyst layers containing a metal catalyst and a fluoric proton-conducting polymer) for a fuel cell, while a study on a polymer solution and a dispersion suitable for such a method is also conducted (refer to Japanese Patent Laying-Open No. 2003-317749 (Patent Document 8)).
Whichever method is employed, it is important to derive the characteristics of the electrodes or the electrolyte membrane in excellent form for the electrode-electrolyte membrane assembly, while material transfer of protons and reaction gas in the electrodes is desirably smooth so that the catalytic performance is excellently derived in relation to the electrodes, and bondability to the electrolyte membrane must also be rendered excellent.
From the aforementioned viewpoint, a method of interposing a composition of a fluoric proton-conducting polymer having a similar structure between electrodes is employed with respect to a conventional fluoric proton-conducting polymer membrane, and a composition or a catalyst ink containing a fluoric proton-conducting polymer suitable therefor is also prepared (refer to Japanese Patent Laying-Open No. 2005-108827 (Patent Document 9), Japanese Patent Laying-Open No. 2000-188110 (Patent Document 10) and Japanese Patent Laying-Open No. 2004-273434 (Patent Document 11), for example). In this case, bondability between the electrodes and the electrolyte membrane can also be maintained excellent due to similar physical properties of the polymers.
While an electrolyte membrane composed of an aromatic hydrocarbon-based proton-conducting polymer is also studied from such a viewpoint that an aromatic hydrocarbon-based polymer electrolyte membrane can stably operate over a longer period when bonded to electrodes holding an aromatic hydrocarbon-based proton-conducting polymer therebetween, the composition interposed between the electrodes is not sufficiently studied. For example, while Japanese Patent Laying-Open No. 2003-317749 (Patent Document 12) shows a composition prepared by dissolving a non-fluoric proton-conducting polymer, this composition is improved in durability when forming an electrolyte membrane by application/formation to/on electrodes (commercially available electrodes containing Nafion (registered trademark) are employed for the electrodes), and a composition according to Japanese Patent Laying-Open No. 2003-249244 (Patent Document 13) is that suitable for forming an electrolyte membrane by casting and not designed to be interposed between electrodes.
Whichever method is employed for preparing an electrode-electrolyte membrane assembly, a fluoric proton-conducting polymer is generally used as the proton-conducting polymer interposed between electrodes also when an electrolyte membrane composed of a non-fluoric proton-conducting polymer is used. It is reported that such an assembly, prepared by bonding different types of polymers to each other, leads to a problem in bondability between the polymers when looked from medium- and long-term perspectives (refer to 205th Electrochemical Society Meeting Abs No. 334 (Non-Patent Document 2)). Further, there has also been room for improvement in relation to homogeneity of catalyst layers.
Patent Document 1: Specification of U.S. Publication No. 2002/0091225
Patent Document 2: Japanese Patent Laying-Open No. 6-93114
Patent Document 3: Japanese Patent No. 3523853
Patent Document 4: Japanese Patent Laying-Open No. 2003-317749
Patent Document 5: Japanese Patent Laying-Open No. 2003-317750
Patent Document 6: Japanese Patent Laying-Open No. 2004-55522
Patent Document 7: Japanese Patent Laying-Open No. 2003-249244
Patent Document 8: Japanese Patent Laying-Open No. 2003-317749
Patent Document 9: Japanese Patent Laying-Open No. 2005-108827
Patent Document 10: Japanese Patent Laying-Open No. 2000-188110
Patent Document 11: Japanese Patent Laying-Open No. 2004-273434
Patent Document 12: Japanese Patent Laying-Open No. 2003-317749
Patent Document 13: Japanese Patent Laying-Open No. 2003-249244
Non-Patent Document 1: Journal of Membrane Science (Netherlands), 1993, Vol. 83, pp. 211-220
Non-Patent Document 2: 205th Electrochemical Society Meeting Abs No. 334